The present invention relates to a process for a numerical control unit to estimate the mass, inertia, viscous friction coefficient and sliding (Coulomb) frictional force of a movable part, e.g. a table of a machine to be controlled. Incorporated herein by reference is the subject matter of Japanese priority application No. 2-93547 filed Apr. 9, 1990.
FIG. 8 is a block diagram illustrating a conventional numerical control unit. In FIG. 8, an interpolation processor 1 is used for entering machining information, e.g. a travel distance and traveling velocity, for each block of a machining program, and for outputting a travel increment per sampling or a position command value of a control axis. An acceleration/deceleration processor 2 receives the interpolation information, e.g. the travel increment per sampling or the position command value provided by the interpolation processor 1, and performs acceleration/deceleration processing for a primary delay circuit, for example, and outputs a position command value for a motor 105 or a travel increment per sampling. A servo controller 3 is responsive to the output of the acceleration/deceleration processor 2 and controls the positioning of the motor 105. The servo controller 3 comprises a position loop controller 101, a velocity loop controller 102, a current loop controller 103, a current detector 104 for detecting a motor current feedback value, the motor 105 for driving a movable part (not illustrated), a velocity detector 106 operatively connected to the movable part (not illustrated) or to the motor 105, and a position detector 107.
In operation, the interpolation processor 1 receives machining information, such as a travel distance and traveling velocity, for each block of the machining program and outputs to the acceleration/deceleration processor 2 a. travel increment per sampling or a position command value of the control axis. The acceleration/deceleration processor 2 receives the interpolation information, such as the travel increment per sampling or the position command value provided by the interpolation processor 1, performs acceleration/deceleration processing for a primary delay circuit having a preset time constant, for example, and outputs a position command value for the motor 105 or travel increments per sampling to the servo controller 3. The servo controller 3 controls the position of the movable part in response to the output of processor 2 by using in sequence the position loop control 101, the velocity loop control 102 and the current loop control 103 to generate operational inputs to the motor 105, in accordance with the position command value or the travel increment per sampling. In this conventional system, the movable part is controlled by using preset loop gains and compensation gains in the position loop control 101, the velocity loop control 102 and the current loop control 103.
In the conventional numerical control unit configured as mentioned above, the time constant of the acceleration/deceleration processor, as well as the loop gains of the position loop control 101, the velocity loop control 102 and the current loop control 103, and each compensation gain, are set while inertia, a viscous friction coefficient or sliding frictional force of the movable part of the machine to be controlled remain unknown or are identified merely as approximate values. In recent years, however, as higher machining speed and higher machining accuracy are demanded, it has become necessary to provide fast, accurate and stable positioning control. Nevertheless, since the conventional numerical control unit carries out positioning control of the movable part without precisely knowing the inertia, viscous friction coefficient and sliding frictional force of the movable part of the machine to be controlled, the acceleration/deceleration time constant, the position loop gain and the velocity loop gain, for example, of the servo controller must be manually adjusted by skilled operators. Even when such operators have a significant level of skill, they require a considerable amount of time to make such adjustments.
The machining of a workpiece having a projection with a curved profile offers a particularly difficult problem. In determining the appropriate machining adjustments when machining a projection which requires switching of circular arc quadrants, the mass and the frictional force of the movable part must be taken into consideration. An accurate offset during high-speed machining cannot be determined by the conventional numerical control unit because it can only compensate for that projection by using the acceleration parameter and the mass of the movable part.